You only need a speed and cadence sensor if that data is important to you (speed sensor is necessary if you plan on field testing). Personally I use a Garmin 920xt with the quick release kit and the bike computer set to display power only for my races.
I am considering buying a power meter, the P1 pedals (only one sided), and have a question. What type of device do I buy to read the power numbers, and is there one that offers a bigger display which is easier to read? I have a Timex bike computer but the screen is very small and not easy to read. I do not have it installed on my road bike.
Do I also need a speed and cadence sensor, or just the power meter and the device to secure to the handlebars?I’d recommend the Wahoo ELMNT. It has a large screen and good contrast.
You don’t need a speed-cadence sensor, as the P1s will provide cadence and the ELMNT has GPS for speed (although it won’t be as accurate as a speed sensor on the wheel).
I use a garmin 810. The numbers are nearly a half inch tall. (I’ll actually measure when I get home)